By Miles Chambers

After confirmation Robert Lewandowski will join Bayern Munich in the summer, BVB have moved to calm fears amongst supporters by promising a summer of grand investment

Borussia Dortmund are willing to splash the cash in the summer to challenge Bayern Munich at the top of the Bundesliga once again, according to sporting director Hans-Joachim Watzke.

After winning back-to-back league titles in 2010-11 and 2011-12, BVB were usurped as champions by the treble-winning Bavarians in 2012-13 and have fallen well behind Pep Guardiola's men this campaign.

Star striker Robert Lewandowski has agreed to join Bayern from Dortmund on a free transfer at the end of the season - less than 12 months since Mario Gotze's move from Signal Iduna Park to the Allianz Arena - with Jurgen Klopp's men in danger of being cut adrift.

But Watzke has promised to fight against the tide by spending big in the summer - but he stopped short of revealing how many millions BVB are willing to part with for top-quality players.

"It's no secret: we will invest a lot of money in the transfer market in the summer," he told Sport Bild. "The amount we're going to spend won't be revealed though."

Dortmund face Augsburg on January 25 in their first Bundesliga games since the winter break, as they look to claw back an 11-point deficit between them and Bayern.